One that I would remove from the list would be Tasmin Drew - not because it's not a televised episode but because she wasn't that much of a henchman in my eyes.

I would include the robots (from "Robots of Death") because when they were reprogrammed they became cold emotionless henchmen to their new boss Taren Capel, that would even kill for him without a second thought.

Another inclusion would be Mr. Sin (from "The Talons of Weng-Chiang"). He was a killer and henchman for the maniacal Magnus Greel in the story and did so with no concern for the consequences to himself.